Add github action for running tests against lowest matching depedencies

[bshaffer]

Add a check to run the tests against the lowest matching dependencies. This can be done with our current test suite, but by providing --prefer-lowest to the command to run composer install. This will ensure that if we have an older dependency which needs to have the minimum version updated to the latest, we'll see that highlighted in our test suite.

Rationale: There have been a few occasions where a required dependency was not declared as such, and resulted in an error for a customer.

[bshaffer]

We just had an error related to this happen again. See googleapis/google-auth-library-php#504 and the resulting fix in #6896